1. Field of Invention
The present invention relates to a light adjustable control circuit, which is directed to a beam density adjustment control circuit using a plurality of LEDs as the light source. More specifically, the present invention relates to a beam density adjustment control circuit using a plurality of LEDs with different colors as the light source.
2. Description of Related Art
Using an LED as the light source, simply utilizing direct current (DC) driving circuit can realize the demand for the LCD backlight or general lighting system. Due to the characteristics, The relationship between the LCD brightness and the driving DC is non-linear, and the LCD color may also vary with the change of driving current. Thus, using LED as the light source for the LCD backlight or general lighting system becomes problematic in the application of light adjustment through adjusting the LED driving DC directly.
To overcome the shortcoming of adjusting light through the current, instead of changing the amplitude of the LED driving current, the practice at present is to adjust the light by using a predetermined working frequency to alter the LED current beam density in the case that the amplitude of the LED current is fixed, so that the LED shows the needed stable color within the maximum light adjusting range.
With reference to FIG. 1 and FIG. 2, FIG. 1 is a schematic diagram of the conventional method of using DC current supply to drive the LED and using the beam density light adjustment to control the output current. FIG. 2 is a schematic diagram of the relation between the brightness control pulse signal and the LED driving DC current signal of the circuit in FIG. 1. In FIG. 1, the brightness control pulse signal CNTL which controls the brightness/dimness of the LED 120 is input to the LED DC current supply 110 to control the LED DC current supply 110 to output an LED driving current signal Id which drives the LED 120. The LED driving current signal Id is a fixed current which sets the current value according to the brightness requirement. In FIGS. 2, (a), (b), (c) are three output timing diagrams of the LED driving current signal Id controlled by different pulse width outputs. FIG. 2(a) is a situation when the brightness is only 20% of its full brightness, FIG. 2(b) is an example when the brightness is only 60% of its full brightness and FIG. 2(c) is an example when the brightness is its 100% full brightness.
To avoid the visual interference to human eyes because of the intermittent lighting and dimming, generally the frequency of the brightness control pulse signal CNTL should not be too low; normally it is above 200 Hz. According to the effect of persistence of vision, the brightness control pulse signal CNTL with the frequency high enough can make human eyes only feel the brightness alternation of LED without flickering.
Since the frequency and the working cycle of the brightness control pulse signal CNTL used in the above description are set based on the required brightness, therefore once the brightness is adjusted, using the frequency of the brightness control pulse signal CNTL in LCD backlight may encounter the beat interference problem generated by the vertical and horizontal scanning signals in the video display signal. Because the backlight and the video signals have different frequencies that causes the so-called “fan effect”, so that results in water ripples on the video images. In addition, the activation and cut-off of the LED DC current supply may also cause the loading on the power supply supplying the needed power for the LED DC current supply, so that generate the ripples with the same frequency with the brightness control pulse signal CNTL in the power supply. These ripples may also affect the video display signal and result in flickering images. In the circumstance of using more LEDs as the light source, the interference caused by the adjustment of the beam density may become severer as the LED operation power increases.
To avoid the interference caused by the different frequencies of the brightness control pulse signal CNTL and the vertical and horizontal scanning signal in the video display signal, there is a method which let the frequency of the brightness control pulse signal CNTL and the horizontal scanning signal be multiplied and synchronously. In addition, the frequency of the brightness control pulse signal CNTL can also be increased to reduce the interference to the power supply caused by formation of the ripples. However, in the trend that the LCD size is getting bigger and bigger, more and more LEDs are used, and the power consumption is getting larger and Larger, and in the circumstance of the requirement for reducing visual noise is getting stricter, it becomes more and more difficult to keep low noise, light adjustment of wide range in practical mass production.